/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you under the Apache License, Version 2.0 (the

  * "License"); you may not use this file except in compliance

  * with the License.  You may obtain a copy of the License at

  *

  *     http://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 package org.apache.giraph.edge;

 import it.unimi.dsi.fastutil.longs.LongArrayList;

 import it.unimi.dsi.fastutil.longs.LongIterator;

 import java.io.DataInput;

 import java.io.DataOutput;

 import java.io.IOException;

 import java.util.Iterator;

 import org.apache.giraph.utils.EdgeIterables;

 import org.apache.giraph.utils.Trimmable;

 import org.apache.hadoop.io.LongWritable;

 import org.apache.hadoop.io.NullWritable;

 /**

  * Implementation of {@link OutEdges} with long ids and null edge

  * values, backed by a dynamic primitive array.

  * Parallel edges are allowed.

  * Note: this implementation is optimized for space usage,

  * but random access and edge removals are expensive.

  */

 public class LongNullArrayEdges

     implements ReuseObjectsOutEdges<LongWritable, NullWritable>,

     MutableOutEdges<LongWritable, NullWritable>, Trimmable {

   /** Array of target vertex ids. */

   private LongArrayList neighbors;

   @Override

   public void initialize(Iterable<Edge<LongWritable, NullWritable>> edges) {

     EdgeIterables.initialize(this, edges);

   }

   @Override

   public void initialize(int capacity) {

     neighbors = new LongArrayList(capacity);

   }

   @Override

   public void initialize() {

     neighbors = new LongArrayList();

   }

   @Override

   public void add(Edge<LongWritable, NullWritable> edge) {

     neighbors.add(edge.getTargetVertexId().get());

   }

   /**

    * If the backing array is more than four times as big as the number of

    * elements, halve its size.

    */

   private void trimBack() {

     if (neighbors.elements().length > 4 * neighbors.size()) {

       neighbors.trim(neighbors.elements().length / 2);

     }

   }

   /**

    * Remove edge at position i.

    *

    * @param i Position of edge to be removed

    */

   private void removeAt(int i) {

     // The order of the edges is irrelevant, so we can simply replace

     // the deleted edge with the rightmost element, thus achieving constant

     // time.

     if (i == neighbors.size() - 1) {

       neighbors.popLong();

     } else {

       neighbors.set(i, neighbors.popLong());

     }

     // If needed after the removal, trim the array.

     trimBack();

   }

   @Override

   public void remove(LongWritable targetVertexId) {

     // Thanks to the constant-time implementation of removeAt(int),

     // we can remove all matching edges in linear time.

     for (int i = neighbors.size() - 1; i >= 0; --i) {

       if (neighbors.getLong(i) == targetVertexId.get()) {

         removeAt(i);

       }

     }

   }

   @Override

   public int size() {

     return neighbors.size();

   }

   @Override

   public Iterator<Edge<LongWritable, NullWritable>> iterator() {

     // Returns an iterator that reuses objects.

     // The downcast is fine because all concrete Edge implementations are

     // mutable, but we only expose the mutation functionality when appropriate.

     return (Iterator) mutableIterator();

   }

   @Override

   public Iterator<MutableEdge<LongWritable, NullWritable>> mutableIterator() {

     return new Iterator<MutableEdge<LongWritable, NullWritable>>() {

       /** Current position in the array. */

       private int offset = 0;

       /** Representative edge object. */

       private final MutableEdge<LongWritable, NullWritable> representativeEdge =

           EdgeFactory.createReusable(new LongWritable());

       @Override

       public boolean hasNext() {

         return offset < neighbors.size();

       }

       @Override

       public MutableEdge<LongWritable, NullWritable> next() {

         representativeEdge.getTargetVertexId().set(neighbors.getLong(offset++));

         return representativeEdge;

       }

       @Override

       public void remove() {

         // Since removeAt() might replace the deleted edge with the last edge

         // in the array, we need to decrease the offset so that the latter

         // won't be skipped.

         removeAt(--offset);

       }

     };

   }

   @Override

   public void write(DataOutput out) throws IOException {

     out.writeInt(neighbors.size());

     LongIterator neighborsIt = neighbors.iterator();

     while (neighborsIt.hasNext()) {

       out.writeLong(neighborsIt.nextLong());

     }

   }

   @Override

   public void readFields(DataInput in) throws IOException {

     int numEdges = in.readInt();

     initialize(numEdges);

     for (int i = 0; i < numEdges; ++i) {

       neighbors.add(in.readLong());

     }

   }

   @Override

   public void trim() {

     neighbors.trim();

   }

 }